1. Field of the Invention
Embodiments of the invention generally relate to analysis of plating solutions, and more particularly, to the analysis of additives in plating solutions.
2. Description of the Related Art
Metallization of sub-quarter micron sized features is a foundational technology for present and future generations of integrated circuit manufacturing processes. More particularly, in devices such as ultra large scale integration-type devices, i.e., devices having integrated circuits with more than a million logic gates, the multilevel interconnects that lie at the heart of these devices are generally formed by filling high aspect ratio interconnect features with a conductive material, such as copper or aluminum, for example. Conventionally, deposition techniques such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been used to fill interconnect features. However, as interconnect sizes decrease and aspect ratios increase, efficient void-free interconnect feature fill via conventional deposition techniques becomes increasingly difficult. As a result thereof, plating techniques, such as electrochemical plating (ECP) and electroless plating, for example, have emerged as viable processes for filling sub-quarter micron sized high aspect ratio interconnect features in integrated circuit manufacturing processes.
In an ECP process, for example, sub-quarter micron sized high aspect ratio features formed into the surface of a substrate may be efficiently filled with a conductive material, such as copper, for example. ECP plating processes are generally two stage processes, wherein a seed layer is first formed over the surface and features of the substrate, and then the surface and features of the substrate are exposed to a plating solution, while an electrical bias is simultaneously applied between the substrate and an anode positioned within the plating solution. The plating solution is generally rich in ions to be plated onto the surface of the substrate, and therefore, the application of the electrical bias causes these ions to be urged out of the plating solution and to be plated onto the seed layer. Furthermore, the plating solution generally contains organic additives, such as, for example, levelers, suppressors, and accelerators that are configured to increase the efficiency and controllability of the plating process. These additives are generally maintained within narrow tolerances, so that the repeatability and controllability of the plating operation may be maintained and repeated.
Monitoring and/or determining the composition of a plating solution during an ECP process is problematic, as the depletion of certain additives is not necessarily constant over a period of time, nor is it always possible to correlate the plating solution composition with the plating solution use. As such, it is difficult to determine the concentration of additives in a plating solution with any degree of accuracy over time, as the level of additives may either decrease or increase during plating, and therefore, the additive concentrations may eventually exceed or fall below the tolerance range for optimal and controllable plating. Conventional ECP systems generally utilize a cyclic voltammetric stripping (CVS) or a cyclic pulse voltammetric stripping (CPVS) process to determine the organic additive concentrations in the plating solution. In a CVS process, for example, the potential of a working electrode is swept through a voltammetric cycle that includes both a metal plating range and a metal stripping range. The potential of the working electrode is swept through at least two baths of non-plating quality, and an additional bath where the quality or concentration of organic additives therein is unknown. In this process, an integrated or peak current used during the metal stripping range may be correlated with the quality, i.e., concentration of additives, of the non-plating bath. As such, an integrated or peak current may be compared to the correlation of the non-plating bath, and the quality of the unknown plating bath determined therefrom. The amount of metal deposited during the metal plating cycle and then redissolved into the plating bath during the metal stripping cycle generally correlates to the concentration of particular organics, generally brighteners or accelerators, in the plating solution. Therefore, CVS methods generally observe the current density of the copper ions reduced on an electrode at a predetermined potential. Inasmuch as accelerators and brighteners increase the current density, the quantity of both may be determined from the observation.
However, one challenge associated with utilizing conventional CVS methods for determining the concentration of organics in a plating solution is that by-products, such as organic contaminants generated in plating processes, may interfere with the analysis process. More particularly, by-products essentially compete with additives for adsorption sites in certain potential ranges, and therefore, if the analysis scanning range includes the by-product adsorption range, the analysis of the unknown additive concentration may be affected by the adsorption of the by-products. Furthermore, the effect of the by-products on the additive analysis is amplified at higher working electrode rotation rates because the by-products and additives diffuse at a faster rate. Another challenge associated with conventional CVS methods is that a wide potential scanning range generally is used to ensure analysis reproducibility, thereby resulting in a relatively long time between analysis and correction.
CPVS processes attempt to overcome the challenges of conventional CVS processes by sequentially pulsing the working electrode between metal plating, stripping, cleaning, and equilibration steps to maintain the working electrode surface in a relatively clean and reproducible condition. CPVS generally avoids the by-product adsorption potential range by pulsing to the known additive adsorption potential range, i.e., by moving directly from an open circuit potential to a potential within the additive adsorption range without scanning through the by-product adsorption range. The steady-state charge density corresponding to the stripping step is then proportional to the additive concentration.
However, CPVS is not without challenges. For example, CPVS generally does not provide control over the rate of the forward reaction for metal deposition. Therefore, separation of interference is difficult. Due to the strong interaction among multiple additives competing for working electrode surface adsorption sites, the analysis of any one single additive may suffer from the interference of the other additives. As such, there is a need for a method for measuring additives in a plating solution, wherein the method is not susceptible to the inaccuracies of conventional analysis processes.
Embodiments of the invention generally relate to a cyclic voltammetric method for measuring the concentration of additives in a plating solution. The method generally includes providing the plating solution having an unknown concentration of an additive to be measured therein, and cycling the potential of an inert working electrode in contact with the plating solution through a series of measurement steps. The series of measurement steps generally includes a metal stripping step, including pulsing from an open circuit potential to a metal stripping potential between about 0.2 V and about 0.8 V, and holding the metal stripping potential until a corresponding current is about 0 mA/cm. The series of measurement steps further includes a cleaning step including pulsing from the metal stripping potential to a cleaning potential between about 1.2 V and about 1.6 V, and holding the cleaning potential for about 2 seconds to about 10 seconds. The series of measurement steps then includes a pre-plating step including pulsing from the cleaning potential to a pre-plating potential between about xe2x88x920.2 V and about xe2x88x920.5 V, and holding the pre-plating potential for about 2 seconds to about 10 seconds. The series of measurement steps may additionally include an equilibration step including pulsing from the pre-plating potential to the open circuit potential and holding the open circuit potential for a predetermined period, and a metal deposition step including scanning from the open circuit potential of the equilibration step to an additive sensitive potential, holding the additive sensitive potential for about 1 second to about 30 seconds, and scanning back to the open circuit potential. The method may further include determining a profile of a deposition current resulting from the metal deposition potential as a function of time and integrating the deposition current to determine the concentration of the additive to be measured.
Embodiments of the invention further provide a method for measuring the concentration of additives in a plating solution. The method generally includes cycling the potential of an inert working electrode through a series of measurement steps. The series of measurement steps generally includes a metal stripping step including pulsing from an open circuit potential to a metal stripping potential sufficient to remove substantially all metal on the working electrode, and holding the metal stripping potential until a corresponding current nears 0 mA/cm. The series of measurement steps then includes a cleaning step including pulsing from the metal stripping potential to a cleaning potential and holding the cleaning potential for about 2 seconds to about 10 seconds. The series of measurement steps further includes a pre-plating step including pulsing from the cleaning potential to a pre-plating potential sufficient to plate a layer of metal on the working electrode, and holding the pre-plating potential for about 2 seconds to about 10 seconds, and an equilibration step including pulsing from the pre-plating potential to the open circuit potential, and holding the open circuit potential for a predetermined period. The series of measurement steps additionally includes a metal deposition step including scanning from the open circuit potential of the equilibration step to an additive sensitive potential, holding the additive sensitive potential for about 1 second to about 30 seconds, and scanning back to the open circuit potential. The method further includes determining a profile of a deposition current resulting from the metal deposition potential as a function of time and integrating the deposition current to determine the concentration of the additive to be measured.
Embodiments of the invention further provide a cyclic voltammetric method for measuring the concentration of additives in a plating solution. The method generally includes cycling the potential of an inert working electrode through a series of measurement steps. The series of measurement steps include a metal stripping step which includes pulsing from an open circuit potential to a metal stripping potential between about 0.4 V and about 0.6 V, and holding the metal stripping potential for about 2 seconds to about 10 seconds, and a cleaning step including pulsing from the metal stripping potential to a cleaning potential between about 1.2 V and about 1.6 V, and holding the cleaning potential for about 2 seconds to about 10 seconds. The series of measurement steps further includes a pre-plating step including pulsing from the cleaning potential to a pre-plating potential between about xe2x88x920.2 V and about xe2x88x920.5 V, and holding the pre-plating potential for about 2 seconds to about 10 seconds, and an equilibration step including pulsing from the pre-plating potential to the open circuit potential, and holding the open circuit potential for about 10 seconds to about 60 seconds. The series of measurement steps additionally includes a metal deposition step including scanning from the open circuit potential of the equilibration step to an additive sensitive potential, holding the additive sensitive potential for about 1 second to about 30 seconds, and scanning back to the open circuit potential. The method further includes repeating the series of measurement steps until steady state is reached, determining a profile of a deposition current resulting from the metal deposition potential as a function of time and integrating the deposition current to determine the concentration of the additive to be measured, and averaging the deposition currents from the series of measurement steps when steady state has been reached to determine the concentration of the additive to be measured.
Embodiments of the invention further provide a cyclic voltammetric method for measuring the concentration of additives in a plating solution. The method generally includes providing the plating solution having an unknown concentration of an additive to be measured therein and cycling the potential of an inert working electrode through a series of measurement steps. The series of measurement steps includes a metal stripping step including pulsing from an open circuit potential to a metal stripping potential between about 0.4 V and about 0.6 V, and holding the metal stripping potential for about 2 seconds to about 10 seconds, and a cleaning step including pulsing from the metal stripping potential to a cleaning potential between about 1.2 V and about 1.6 V, and holding the cleaning potential for about 2 seconds to about 10 seconds. The series of measurement steps further includes a pre-plating step including pulsing from the cleaning potential to a pre-plating potential between about xe2x88x920.2 V and about xe2x88x920.5 V and holding the pre-plating potential for about 2 seconds to about 10 seconds, and an equilibration step including pulsing from the pre-plating potential to the open circuit potential, and holding the open circuit potential for about 10 seconds to about 60 seconds. The series of measurement steps additionally includes a metal deposition step including scanning from the open circuit potential of the equilibration step to an additive desorption potential and holding the desorption potential for about 1 second to about 30 seconds, scanning to an additive adsorption potential and holding the adsorption potential for about 5 seconds to about 30 seconds, and scanning from the additive adsorption potential to the open circuit potential. The method further includes repeating the series of measurement steps until steady state is reached, determining a profile of a deposition current resulting from the metal deposition potential as a function of time and integrating the deposition current over the additive desorption potential to determine the concentration of the additive to be measured, and averaging the deposition currents from the series of measurement steps when steady state has been reached to determine the concentration of the additive to be measured.
Embodiments of the invention also provide a cyclic voltammetric method for measuring the concentration of additives in a plating solution. The method generally includes the step of cycling the potential of a working electrode through a series of measurement steps, comprising a metal stripping step, a cleaning step, a pre-plating step, an equilibration step, and a metal deposition step. The method also may include the step of determining a profile of a deposition current resulting from the metal deposition potential as a function of time and integrating the deposition current to determine the concentration of the additive to be measured.